You've really set up a challenging set of circumstances:
1. 62 acres
2. mow 15-20 acres
3. driveway maintenance
4. building house
5. $18k to $19k total.
6. $2k to $3k for trailer
7. 4wd
8. loader
9. lots of trees
10. little (5') rotary cutter
11. boxblade or rear blade
12. etc., etc., etc.
Wow! You don't need a tractor, you need a bigger wallet. /forums/images/graemlins/shocked.gif /forums/images/graemlins/smile.gif
Okay, I think your budget is gonna get busted. For a new tractor, you need to start about $25k. That will get you into the range you really need. Of course, if you are serious about the small cutter and just a little boxblading, your 30 HP tractor might be okay. Just set aside a lot of time for your jobs because you are going to get a lot of seat time with a small tractor and cutter.
Here are some things to think about. How much work will you do in the trees? If doing a lot, a high exhaust pipe, canopy, or cab may not be a wise choice. Keep your tractor as low and compact as possible to fit into as many tight spots as you are likely to find.
40 hp is a lot more than 30. You might not feel a major difference with a 5 HP difference, but you will notice a big difference in the weight and feel of the larger tractor with 10 more HP.
Do you plan to lift loads over 1,000#? A 40 HP tractor will let you lift 3PH loads of over a ton and loader lift of 3/4 ton. At 30 HP you will not have nearly the lift nor the stability of the larger/heavier tractor. If you want serious lifting above a ton on both ends, you need to go above the compacts to the the 50-65 HP range.
Your challenges with 62 acres are going to be plenty. If I absolutely only had $18k to spend. I look for used tractors and try that route first for the best tractor I could find. If you must have a new tractor, I'd look at the bare bones tractors like the NH TC30 and JD7xx.
To me, your decisions and choices are a lot tougher than buying a car or truck. Remember, you bought a Dodge 3500 instead of a 1500 for a reason. Buying the right tractor is just as important and may be around longer than your truck. You just can't go wrong buying the right tractor to fit your job.